The present invention relates to a test instrument which includes testing circuitry which can be inserted between the input and output sides of telephone subscriber lines for indicating wiring and station equipment faults on the telephone lines and determining the direction of such faults. It is an interface between the telephone supply company and the equipment and circuits which the customer provides. This instrument provides means whereby the user can test that portion of the circuit which he is required to maintain and also check the input to the circuit for proper voltage and polarity. By means of the present instrument, there is provided an indication of the most common wiring faults which occur, such as grounds, foreign E.M.F., resistance across the lines, open circuits, etc. A feature of the present instrument is that it may be portable or wired and requires no internal power source. It uses light emitting diodes to provide convenient visual display of any faults detected.
As the telephone industry has been changing in recent times, it has become common for customers to use more of their own equipment and to do a greater amount of their own wiring, instead of relying on the telephone company. There exists a need for devices which can aid and assist the relatively unskilled and unprofessional people in recognizing problems which can affect their telephone service. These people should be able to identify the source of the problem and correct it or, at the very minimum, be able to identify those areas which may be responsible for causing the fault and determine what they believe to be the appropriate procedure in order to correct the fault. This would eliminate many calls to the telephone company for service, for which the customer is billed. Another problem is that on many occasions when the problem is not the responsibility of the telephone company, the service is not restored and the problem remains.
At the present time devices which are used to identify faults on telephone lines are intended primarily for use by professional or skilled operators. These devices typically include a variety of devices such as meters, LED indicators and opening demarkation blocks. The meters usually require knowledge of their operation and interpretation of the measured results in order to use them properly. The LED indicators which are normally employed are inserted into the line and test for various conditions at that point. The indicators cannot segregate sections of the circuit so that the particular location of the fault is well defined, nor do they indicate the direction of the fault. These indicators commonly require manipulation of various connections in order to obtain an accurate indication of the fault, which in turn requires a keen knowledge and certain skill to attain that result.
Other typical prior art is exemplified by the U.S. Pat. Nos. 3,920,933 (Moorehead); 3,941,950 (Dunwoodie et al); 3,944,914 (Simmons); 4,022,990 (Bauer); and 4,144,487 (Pharney). The Dunwoodie et al patent discloses a portable telephone test instrument, including the use of LED's to indicate certain characteristics of the tested circuit. There is a visual indication of the presence and polarity of direct current in the line under test. An LED will provide an indication when a proper connection has been made through the central office switching equipment. The device enables the communicating state of the telephone circuit under test to be monitored while simultaneously conducting a quality test of the circuit. External power is supplied to the device. In Simmons the device detects the existence of physical faults such as shorts, grounds, opens and the like. The test circuit periodically electrically charges and discharges each line to determine the distributed capacitance of each pair, which is then compared with the capacitance of other pairs of conductors to reveal any faults in the conductors. The Bauer test apparatus enables determination of an open or break in a wire of the telephone cable and provides information as to the location of the open condition on the cable. The Pharney apparatus locates faults such as high resistance leaks in telephone lines and simultaneously repairs the fault by means of a weld created between the conductors at the fault location.
Notwithstanding the foregoing prior art, there still exists a need for relatively simple, self-contained, portable test instrument which preferably uses power supplied by the telephone lines under test and provides an indication of, among other things, the direction of the fault.